DESCRIPTION (investigator's abstract): The broad, long-term objective of the proposed research is to characterize the biochemical processes and immunological importance of a novel antigen processing and loading pathway recently shown to utilize empty or "peptide-receptive" cell-surface MHC class II proteins in certain professional antigen cells. The proposed research is intended to determine the relationship between MHC class II stability and expression of empty molecules on the surface of dendritic cells, to investigate the expression and functional capacity of empty class II MHC molecules on central nervous system microglia cells, to identify the protease(s) involved in extracellular antigen processing in immature dendritic cells, and to evaluate the importance of extracellular versus intracellular antigen loading pathways in dendritic cells and microglia. These goals will be achieved through biochemical characterization of empty and peptide-loaded MHC molecules using antibody and peptide binding assays, purification and characterization of secreted protease activity, and the use of specific inhibitors of intracellular and extracellular processing pathways. The proposed research is part of an interactive research project group. The overall objective of the IRPG is to characterize a novel pathway for antigen processing and presentation involving extracellular proteases and empty class II MHC proteins that has been identified initially in immature dendritic cells. This project supports that goal by focusing on biochemical aspects of antigen processing and peptide loading in dendritic cells and microglia. The companion project focuses on MHC trafficking and antigen presentation.